Uranus and Neptune's weird magnetic fields are unlike Earth's, and now U.S. researchers have explained why.

Instead of having magnetic fields like bar magnets through the poles, the magnetic fields of these outer planets have no poles and are inclined at a crazy angle.

The researchers used a mathematical model that showed the circulating layer that creates these magnetic field is actually in the planets' thin crusts, rather than next to the core, like it is in the Earth.

Sabine Stanley and Professor Jeremy Bloxham from Harvard University published their results today in the journal Nature.

Most planets have a dipolar magnetic field, with a north and south pole, that centres on an imaginary line or axis between the poles.

Earth's magnetic field is inclined at 11º to the pole, which is why the magnetic north is at a slightly different place to the geographical North Pole.

Many other planets are similar, including the giant planets Jupiter and Saturn, as well as Jupiter's moon Ganymede, and probably Mercury. Jupiter, for example, has a magnetic field inclined at 10º, similar to Earth's.

But Neptune and Uranus have magnetic fields that are way off, inclined at 47º and 59º respectively.

Scientists 10 years ago suggested that this could be because there were circulating currents in the planets' thin crusts. The crusts are electrically charged fluid 'ice' made of water, methane, ammonia and hydrogen sulfide.

Now Stanley and Bloxham have tested this theory and shown that the circulation or convection in the thin icy crust is indeed the cause of the planets' bizarre magnetic fields, because this is the part of the planets that is fluid and in motion.

The researchers said that the magnetic fields were generated by complex fluid motions in electrically conducting regions of the planets, a process known as dynamo action.

The researchers' model produced a planet with a magnetic field highly inclined to the poles, just like Voyager 2 had first observed in Neptune and Uranus.

Not all poles are equal

Australian geomagnetism expert Dr Ted Lilley from the Australian National University in Canberra said the research was significant but not surprising.

"The more remarkable is that the Earth's magnetic field is dipolar from where we stand on the surface and approximately axially symmetric. That's why we expect that on other planets," said Lilley.

In the Earth, the motion of the liquid outer core creates the magnetic field. Although we generally associate magnetism with iron, any electrically charged fluid in motion could create a magnetic field, Lilley said.

"It just depends on where the planet happens to have fluid for dynamo action to occur," he said

"In Earth, it happens to be the outer core. These other two planets might have no fluid, or they might be fully fluid. But they both seem to have a layer that is fluid, is a good conductor of electricity, and is being driven in motion by something or other. And this is what you need to exhibit dynamo action."

As the dynamo action is taking place in a different part of the Uranus and Neptune than it did on Earth, it was not surprising that they had such different magnetic field, he said.

"It is because [on these planets] the conditions were appropriate to create a liquid rather than a solid crust."